#include<bits/stdc++.h>
using namespace std;
int main(){
	freopen("stairs.in" , "r" , stdin);
	freopen("stairs.out" , "w" , stdout);
	int a[35] , n;
	cin >> n;
	a[1] = 1;
	a[2] = 2;
	for(int i = 3 ; i <= n ; i++){
		a[i] = a[i - 2] + a[i - 1];
	}
	cout << a[n] << endl;
	return 0;
}